Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output shaft driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns. Transmissions also may include a start device, such as a torque converter or start clutch, which is capable of transmitting torque from the engine to the input shaft when the input shaft is not rotating. A number of gearing elements, such as planetary gear sets, are interconnected with the input shaft, output shaft, and housing by a number of shift elements such as brakes and clutches. Various transmission speed ratios are established by selectively engaging various subsets of the shift elements.
In typical front wheel drive vehicles, the transmission and differential are combined into a unit called a transaxle. It is common for the gearing elements that establish the various ratios to drive an output gear that rotates about the input shaft and is physically located between the gearing elements and the start device. Conventionally, the output gear is supported via internal bearings by a bell housing that separates the gearing elements from the start device. Torque and power is transmitted from the output gear to the differential via transfer gears that rotate about a transfer axis. The vehicle engine compartment, which must contain both the engine and the transaxle, has a limited width. Therefore, it is desirable to minimize the length of the transaxle. Also, it is desirable to be able to locate the differential as close as possible to the center of the vehicle.